The present invention relates to a method for forming a hydraulic cement coating on the interior surface of a hollow cylinder such as a pipe.
In the past, cement or similar coatings have been formed on the interior surfaces of metallic pipes by applying the wet cement composition to the interior surface and allowing it to set or harden. Due to the wet condition of the applied coating material, however, the lining has a tendency to slump or sag prior to hardening thereby resulting in non-uniform and inconsistent coatings. Moreover, coatings applied in this fashion have a tendency to crack when cured due to non-uniform contact of the cement coating with the interiorly curved surface to which it is applied and the tendency to slump, sag or pull away from the surface prior to hardening.
It has also been proposed to coat surfaces with cement according to the "Gunite" method. This method involves separately spraying the dry cement mix and water in such a manner that the particles of cement composition and droplets of water intermix in the space between the respective spray nozzles and the surface to be coated. The cement is thoroughly wetted before it reaches the surface where it is allowed to set and cure. Inasmuch, however, as the mixture which reaches the surface to be coated is wet it is difficult to control the uniformity and smoothness of the coating. Moreover, the Gunite method suffers from the disadvantage of "overcoating" whereby, due to "splashing" of the wetted mixture as it hits the surface, irregular peaks or mounds are formed in the coating.
For a description of the pneumatic or spray methods for coating surfaces with cement or mortar and the disadvantages associated therewith see "Concrete Technology and Practice", Taylor, pp 352-355 (Am. Elsevier Pub. Co., N.Y. 1972) and "ACI Manual of Concrete Inspection", pp 206-211 (Am. Concrete Institute Publication SP-2, Detroit, Mich., 1972). In the latter publication it is indicated at page 209 that the quality of the resultant coating is highly dependent upon the skill of the operator of the spray machinery and that constant inspection is required to ensure against incorporation of "rebound" concrete into the work.
It is an object of the present invention to provide a cement coating for the interior surfaces of hollow cylinders such as pipes which do not slump or sag and which have a greatly reduced tendency to crack after curing.